org.el ("org-loaddefs.el"): use `load', not `org-load-noerror-mustsuffix'

* org.el ("org-loaddefs.el"): Load org-loaddefs.el before
requiring any org library. Also use `load', not
`org-load-noerror-mustsuffix'.
(org-effort-durations): Move up to fix a compiler warning.
This patch should reduce complexity without breaking compatibility
with XEmacs. Before this patch, users with a wrong load path would
require org-compat.el from this wrong load path, and loading
org-loaddefs.el would not work.